To generate a token you should choose a user who has enough privileges to use the service.
To create a user and assign the role, please follow these steps:
1. Create a new user
Go to Site administration - Users - Accounts - Add a new user
2. Create a new role
Go to Site administration - Users - Permissions - Define roles - Add a new role
a) Upload this XML file with required permissions; or
b) Add the permissions manually. Please find the list of required capabilities for the service which the user should have HERE.
3. Assign role
Go to Site administration - Users - Permissions - Assign system roles
a) Click the role you have created
b) Move the user you have created to the left side